Katelyn with her fellow Gur, all members of a group of monster hunters lead by Ulma, are looking for their children. The kids have been kidnapped by Cazador's spawn, and the nomads have camped in Rivington trying to mount a rescue or seek revenge.
Katelyn along with other Gur can be found in the Gur Camp attending a funeral for the fallen from a recent offence on Cazador's palace. Their leader Ulma will ask the party to help in defeating Cazador and discovering the fate of their stolen children.
Once Cazador is defeated, Katelyn Lerskin, along with Ulma and the other Gur can be found near the waypoint in Cazador's Dungeon. She will attack the party if the ritual held in the dungeons was completed.
If Astarion had been turned in to Gandrel in the Act One and kidnapped, then at the Act Three Katelyn along with all her fellow Gur (except Gandrel) will be dead.